Re: [Acme] WG last call for draft-ietf-acme-email-smime-06

Ryan Sleevi <ryan-ietf@sleevi.com> Sun, 03 May 2020 05:36 UTC

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From: Ryan Sleevi <ryan-ietf@sleevi.com>
Date: Sun, 3 May 2020 01:36:08 -0400
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To: Ben Schwartz <bemasc=40google.com@dmarc.ietf.org>
Cc: Alexey Melnikov <alexey.melnikov@isode.com>, "Salz, Rich" <rsalz@akamai.com>, "acme@ietf.org" <acme@ietf.org>
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Subject: Re: [Acme] WG last call for draft-ietf-acme-email-smime-06
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On Sat, May 2, 2020 at 2:11 PM Ben Schwartz <bemasc=
40google.com@dmarc.ietf.org> wrote:

>
>
> On Sat, May 2, 2020 at 8:35 AM Alexey Melnikov <alexey.melnikov@isode.com>
> wrote:
>
>> Hi Ben,
>> On 21/04/2020 01:12, Ben Schwartz wrote:
>>
>>
>> On Wed, Apr 1, 2020 at 5:40 AM Alexey Melnikov <alexey.melnikov@isode.com>
>> wrote:
>>
>>> Hi Ben,
>>>
>>> My apologies for missing your email in March:
>>>
>>
>> And mine for this delayed response.
>>
>>> On 12/03/2020 20:42, Ben Schwartz wrote:
>>>
>>> Section 3 says token-part1 "contains at least 64 bit of entropy", but
>>> Section 3.1 says token-part1 "MUST be at least 64 octet long after
>>> decoding".  Is this difference deliberate?
>>>
>>> No, I obviously made a typo when saying octets. I will fix.
>>>
>> Fixed.
>>
>> Also 64 octets of entropy is a _lot_.  RFC 8555 says "the token is
>>> required to contain at least 128 bits of entropy".
>>>
>>> The draft seems to be oriented entirely toward use with e-mail clients
>>> that have a built-in ACME-S/MIME client.  I'm a bit disappointed that the
>>> draft doesn't accommodate users with "naive" email clients very well, e.g.
>>> by allowing customized subject lines.
>>>
>>> Actually, I was trying to accommodate naive email clients, but it was a
>>> fine balance trying to specify minimal requirements.
>>>
>>> Can you suggest some specific text to change and then we can discuss
>>> whether or not it should be done? My thinking about the Subject header
>>> field was that I wanted to have a unique subject (so that ACME email
>>> messages are easily findable). I also wanted to allow the token in the
>>> subject for APIs that can easily access Subject and not other header fields.
>>>
>> In that case, I would suggest "... subject ending with "(ACME:
>> <token-part1>)", where ...".  That would allow the first part of the
>> subject (most likely to be seen by a human) to be human-readable.
>>
>> After thinking a bit more about this:
>>
>> As ACME servers are generating ACME challenge emails, the requirement on
>> them is stricter (they create the first message in an email thread). I am
>> tempted to leave this as is. Can you think of a case where ACME servers
>> would be unable to comply with this restriction?
>>
> My concern is that users will not know what to do if they receive an email
> whose subject line is "ACME: awlkNSdpijawrfz...".  Users are used to seeing
> emails whose subject line is "Please verify your email address" or "Confirm
> your email".  (My inbox is full of them.)  I see no reason to disallow that
> here.
>
> Mandating that the subject line be non-human-readable seems like an
> unnecessary barrier to adoption.
>

Are you expecting humans to be the primary interaction point? It almost
seems that ensuring human unfriendliness is a feature, not a bug, towards
the goal of automation.

This structure seems especially important if it has a chance to be adopted
by publicly trusted CAs. One of the big concerns with existing validation
approaches is bodies that are rich-text with links used for approval, and
for which anti-spam or scanning engines inspect (“click”) and cause
improper authorizations. The more structure, the better, towards preventing
accidental authorizations.

ACME responses already allow arbitrary prefix to accommodate naive clients.
>>
>> Similarly, for Section 3.2. Point 6, I would relax the requirement to
>> state that this block must appear somewhere in the body.  That way, if the
>> user sees the response message, it can provide some explanation of what is
>> going on.
>>
>> Good idea. Changed.
>>
>> For Section 3.1 Point 5, I don't understand why the body is restricted to
>> text/plain.  In particular, I think hyperlinks to explanations and
>> instructions are likely to be helpful.  I also wonder whether support for
>> multipart/multilingual could be useful.
>>
>> The body is irrelevant to ACME-aware clients, so it seems like there
>> could be a lot of freedom in how this is constructed.
>>
>> This is true for the challenge email.
>>
> Yes, that's what I was referring to.
>
>> There is a requirement on S/MIME (if used) to provide header protection,
>> but I agree that otherwise the body structure can be pretty flexible.
>>
>> Most email clients automatically convert HTTPS URLs to hyperlinks, which
>> should make the silly schemes I'm imagining possible, but not very
>> attractive, for ordinary users.
>>
>>> Best Regards,
>>>
>>> Alexey
>>>
>>> I assume this is deliberate, perhaps because of a desire to use
>>> short-TTL S/MIME certificates that would be impractical to provision
>>> manually, but the draft doesn't mention a rationale.
>>>
>>> On Thu, Mar 12, 2020 at 2:52 PM Salz, Rich <rsalz=
>>> 40akamai.com@dmarc.ietf..org <40akamai.com@dmarc..ietf.org>> wrote:
>>>
>>>> This mail begins a one-week working group last call on
>>>> https://datatracker.ietf.org/doc/draft-ietf-acme-email-smime/?include_text=1
>>>>
>>>>
>>>>
>>>> If you have comments or issues, please post here.
>>>>
>>>>
>>>>
>>>> If anyone wants to be a document shepherd, please contact the chairs.
>>>>
>>> Best Regards,
>>
>> Alexey
>>
>>
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